In recent years a liquid crystal display device has been widely used in portable calculators, electronic watches, instruments, and so forth. With such a widespread use, the following have further been desired: a reduction in weight and thickness of devices, easy reading, low production costs, and so forth.
Under such circumstances, various attempts have been made to replace conventional liquid crystal display glass cells using a glass plate with plastic cells using a plastic film or sheet.
Such a plastic cell is fabricated by disposing a polarity of cell substrates comprising a polarizing plate which comprises a polarizer and a plastic film or sheet laminated on both surfaces of the polarizer, and an electrode pattern provided on the surface of the polarizing plate, in such a manner that the absorption axis directions of the polarizers are at right angles to each other and the electrode patterns are positioned exactly in a face-to-face relation, and then introducing a liquid crystal between the cell substrates.
It is required for the plastic films in contact with a liquid crystal to have chemical resistance (resistance to liquid crystals) and physical characteristics. Commercially available biaxially stretched films are not satisfactory in respect of optical characteristics although they satisfy the above requirements. On the other hand, unstretched films are satisfactory in optical characteristics but fail to meet the above requirements.
As a plastic film satisfying both the requirements, a monoaxially stretched film is now under investigation. Since, however, it is essential for plastic film to pass through drying and moisture-absorbing steps in the course of production of cell substrates, a satisfactory liquid crystal display plastic cell using such a monoaxially stretched film has not yet been developed.
A cell substrate is produced by the steps of laminating a monoaxially stretched film on both surfaces of a polarizer by bonding techniques such as use of an adhesive or melt adhesion to thereby produce a polarizing plate, and then providing an electrode pattern on the surface of the polarizing plate. During the process of forming such an electrode pattern on the polarizing plate, drying and moisture-absorbing treatments are applied, resulting in deformation of the cell substrate. For this reason, in a liquid crystal display plastic cell fabricated by assembling the cell substrates in such a manner that the absorption axes of the polarizers are at right angles to each other, the electrode patterns of the cell substrates are not positioned exactly in a face-to-face relation; i.e., they are deviated in position from each other, and a sharp display cannot be obtained.
In polarizing plates having the above-described structure, the moisture-absorbing expansion coefficient in a direction (A) in which the monoaxially stretched film is stretched is different from that in a direction (B) perpendicular to the direction (A). That is, the moisture-absorbing expansion coefficient in the direction (B) is greater than that in the direction (A). This means that the polarizing plate is anisotropic in the moisture-absorbing expansion coefficient.
It is essential for cell substrates constituting a liquid crystal display cell that the absorption axes of polarizers constituting the upper and lower cell substrates be at right angles to each other. When, therefore, a liquid crystal display cell is produced by assembling the cell substrates comprising a polarizing plate with an electrode pattern provided thereon, the polarizing plate being produced by laminating monoaxially stretched films on a polarizer in such a manner that the absorption axis of the polarizer is in a parallel relation with a stretching direction of the monoaxially stretched film, i.e., a direction in which the film is stretched, the stretching directions of the monoaxially stretched films constituting the upper and lower cell substrates are at right angles to each other.
In forming an electrode pattern on a polarizing plate, a transparent electrically conductive layer is formed on the surface of the polarizing plate by depositing a transparent thin layer-forming material such as indium oxide containing tin oxide or tin oxide containing antimony, by techniques such as vacuum deposition, sputtering or ion plating, a light-sensitive resin is coated on the transparent electrically conductive layer in a desired pattern, exposed to light, and then subjected to treatments such as chemical etching with acids or alkalis and rinsing to obtain the desired electrode pattern. Since, as described above, a polarizing plate is anisotripic in the moisture-absorbing expansion coefficient, a cell substrate passing through the steps of layer-deposition and moisturing (e.g., rinsing) is inevitably deformed. For this reason, if a liquid crystal display cell is assembled by disposing such deformed cell substrates in such a manner that the absorption axes of the polarizers are at right angles to each other, the electrode patterns of the upper and lower cell substrates are deviated from each other. Thus, a satisfactory display cannot be obtained.